Electrochemical cells in which a chemical reaction is forced by adding electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
A typical electrochemical cell will have a positively charged anode and a negatively charged cathode. The anode and cathode are typically submerged in a liquid electrolytic solution which may be comprised of water and certain salts, acids or base materials. Generally speaking, the anode and cathode are made of a substrate material, such as titanium, graphite, or the like, coated with a catalyst such as lead dioxide or other known materials. The selection of a substrate and catalyst is determined by the particular electrode reactions which are to be optimized in a given situation.
The cathode and anode are positioned within the electrolytic cell with electrical leads leading to the exterior. The cell may be provided with appropriate plumbing and external structures to permit circulation of the electrolyte to a separate heat exchanger. Suitable inlet and outlet passages may also be provided in the cell head space to permit the withdrawal of the gases evolved from the cathode (if gases are to be evolved) and from the anode.
In order to maintain or reduce the temperature of the cell electrodes, heat exchange passages may be provided within the electrode structures. These coolant passages are connected to external sources of coolant liquid which can be circulated through the electrodes during the electrolysis process in order to maintain or reduce their temperatures.
In order to drive the electrolysis reaction, it is necessary to apply electric power to the cell electrodes. The electrodes are connected through the electrical leads to an external source of electric power with the polarity being selected to induce the electrolyte anion flow to the anode and the cation flow to the cathode.
U.S. Pat. No. 3,134,697 (Niedrach) teaches a combination electrode structure and electrolyte for a fuel cell comprising a hydrate, ion exchange resin membrane having a gas adsorbing metal electrode integrally bonded and embedded in each of its two major surfaces. The assembly of the ion exchange membrane and electrode(s) is placed between platens and subjected to sufficient pressure, preferably at an elevated temperature, to cause the resin in the membrane to flow.
U.S. Pat. No. 4,375,395 (Foller) teaches that anodes made of glassy carbon are suitable for use in the preparation of ozone in an electrolytic cell utilizing an aqueous solution of the highly electronegative fluoro-anions.
U.S. Pat. No. 4,541,989 (Foller) teaches that a liquid electrolyte containing acids of fluoro-anions, such as HBF.sub.4 and HPF6, used in combination with a cool electrolyte solution can increase the efficiency and the ozone to oxygen yield. However, the use of a liquid electrolyte causes some problems. First, the electrodes in such electrolytic cell must be separated by a given distance to provide definition. This translates into power loss in the production of heat. Secondly, the presence of liquid electrolytes requires a sophisticated system of seals to prevent leaking of the electrolyte.
U.S. Pat. No. 4,836,929 (Laumann et al.) teaches the use of a solid electrolyte such as that made by dupont and sold under the brand NAFION (NAFION is a trademark of the dupont de Nemoirs Company of Wilmington, Del.). This solid perfluorinated sulphonic acid electrolyte was placed between a lead dioxide anode and a platinum black cathode. The current efficiency was increased by oxygenating a water stream fed to the anode and the cathode. In this manner, oxygen could be reduced to water at room temperature releasing an increased yield of ozone.
In his paper entitled "Synthesis of Hydrogen Peroxide in a Proton Exchange Membrane Electrochemical Reactor" (April 1993), Fenton disclosed an electrochemical reactor that used a membrane and electrode assembly (M&E) comprised of a NAFION 117 membrane between the platinum black/polytetrafluoroethylene (PTFE) anode and graphite/PTFE cathode. This M&E assembly was sandwiched between a carbon fiber paper (Toray Industries) on the cathode side and a platinum mesh (52 mesh, Fisher Scientific) on the anode side which were used as current collectors.
Increasing the percentage of PTFE in the electrode increases the hydrophobicity of the electrode assembly and thus allows more of a gaseous reactant to reach the electrode surface by repelling the products formed. The graphite M&E with 20% PTFE produced slightly greater hydrogen peroxide than a similar M&E with 10% PTFE. This could be due to the mass transport limitation of oxygen to the membrane and electrode assembly within the less hydrophobic 10% M&E. It is preferred that the proton exchange membrane reactor operate at potentials greater than 3.0 volts where the anodic evolution of ozone is favored.
NAFION is a sulfonic acid membrane sold by E. I. dupont Company having a substantially fluorinated backbone and pendant groups according to the following structure: EQU --OCF.sub.2 CF(CF.sub.3)--0--CF.sub.2 CF.sub.2 SO.sub.3 H
Both NAFION 117 (purchased from Aldrich Chemical Company) and NAFION 115 have equivalent weights of 1100 and thicknesses of 7 mils (175 .mu.m) and 5 mils (125 .mu.m), respectively.
U.S. Pat. No. 4,417,969 (Ezzell et al.) discloses ion exchange membranes having sulfonic acid groups. The membrane is a polymer having a substantially fluorinated backbone and recurring pendant sulfonic acid groups represented by the following general formula: EQU --O(CFR.sup.1).sub.b (CFR.sup.2).sub.a SO.sub.3 Y
where a and b are independent integers from zero to three with the condition that the sum of a and b must be at least one; R.sup.1 and R.sup.2 are independently selected from the group consisting of a halogen and a substantially fluorinated alkyl group having one or more carbon atoms; and Y is hydrogen or an alkali metal.
Membranes containing perfluorinated sulphonic acids are typically prepared before use in an electrochemical cell by first soaking the membrane in hot water for about 30 minutes and then soaking it in a mineral acid solution such as10% HCl to ensure that the entire membrane is in the proton (H.sup.+) form. The membrane has to be kept moist at all times since it acts as a conductor only when it is wet.
U.S. Pat. No. 4,416,747 (Menth et al.) discloses an individual electrolysis cell bounded by bipolar plates and having a solid electrolyte made of a polymer of perfluorinated sulphonic acid (NAFION by dupont) with a surface coating centrally located between current-collectors and adjoining open metallic structures. A plurality of individual cells may be integrated together between end plates so that the cells are electrically connected in series, hydrodynamically connected in parallel, and combined to form a block.
U.S. Pat. No. 4,876,115 (Raistrick) discloses a gas reaction fuel cell utilizing porous gas diffusion electrodes formed of carbon particles defining interstitial spaces which expose a catalyst that is supported on the carbon. A solution containing a proton conducting material, such as a perfluorocarbon copolymer, is dispersed over the surface of the electrode to coat the carbon surfaces adjacent the interstitial spaces without impeding gas flow into the interstitial spaces. In this manner, the proton conducting material enables protons produced by the gas reactions adjacent the supported catalyst to have a conductive path with the electrolyte membrane and the carbon particles enable the electrons to have a conductive path to the external circuit.
U.S. Pat. No. 5,242,764 (Dhar) discloses that by depositing a proton conducting material, such as a perfluorocarbon copolymer, on the catalytic side of gas diffusion electrodes, acting as anode and cathode, it is possible to avoid the use of an electrolyte membrane.
U.S. Pat. No. 5,246,792 (Watanabe) discloses an ion exchange membrane having a thin ion-conductive layer with a lower glass transition temperature than that of the ion exchange membrane applied on at least one surface of the membrane. The thin layer, comprising a solution such as a perfluorocarbon copolymer in cyclohexanol, is hot pressed between the membrane and the electrode at a temperature between the glass transition temperatures of the membrane and the thin layer.
The need for water to support proton conduction in the membrane has been addressed in a number of ways. In fuel cells it would initially appear that since water is the product, sufficient water would be already present. Unfortunately, the water formed in a fuel cell is inadequate to maintain membrane hydration except under low current density conditions. Each proton that moves through the membrane drags at least two or three water molecules with it. As the current density increases the number of water molecules moved through the membrane also increases. Eventually the flux of water being pulled through the membrane by the proton flux exceeds the rate at which they return by diffusion. At this point the membrane begins to dry out, and its internal resistance increases. This sets a relatively low limit on the current density that can be maintained by back diffusion from the cathode surface.
This problem has typically been addressed by adding water, as vapor, to the hydrogen containing stream, or to both gas streams (fuel and oxidizer). There is no doubt that this method works, and high power densities can be achieved. Unfortunately, adding a humidifier to the cell stack adds to the size, weight and complexity of the system. If this can be avoided, it would be a great improvement.
A solid state hydrogen pump has all of the same problems as a fuel cell, without the presence of a water forming reaction at the cathode. As with a fuel cell, eliminating the need for a humidifier will lead to a smaller, simpler, and lighter system.
An electrolyzer, especially one designed to produce hydrogen and oxygen from water offers a different set of problems. A water electrolyzer contains essentially the same parts as a fuel cell, but the polarity is reversed, as are all of the electrochemical reactions. Instead of generating electricity and water from hydrogen and oxygen, it produces hydrogen and oxygen from water and electricity.
In an electrolyzer there is always water present to keep the membrane hydrated. The problems arise in the electrodes and on the gas outlet side. Because liquid water is present in the same compartment that gas is being generated in, the gas outlet flow will nearly always be two phase with a large quantity of water being carried out with the gas.
A more fundamental problem arises in the electrodes. Since maximum current efficiency requires that liquid water be in contact with the membrane, at least one of the electrodes must be hydrophilic. While a hydrophilic electrode is best for the membrane, it tends to impede gas bubble formation and gas removal. If the water is supplied directly to the membrane, fully hydrophobic electrodes could be used, to maintain efficient gas evolution.
These problems are further exacerbated in a regenerative fuel cell. Since a regenerative fuel cell by definition must operate in turn as both an electrolyzer and a fuel cell, using hydrophilic electrodes that produce effective operation in a liquid water environment for electrolyzer operation virtually guarantees electrode flooding during fuel cell operation. If operation with liquid water present in the electrode compartment can be avoided, then hydrophobic electrodes can function well in both modes.
One method that has previously been proposed for directly humidifying a proton exchange membrane is the inclusion of water conducting wicks as part of the membrane structure. While this method has some effectiveness, the amount of flow that can be achieved through the membrane is limited. A further drawback to the wicks is that they rely on wetting to promote flow. This precludes their use to introduce non-aqueous streams into the proton exchange membrane.
In addition, the wicks act as filtering elements to remove any particles in the stream. This limits their use to systems with pure water, or where care is taken to prevent the solution from becoming saturated and beginning to precipitate.
Electrochemical water desalination or clean-up systems based on the electroosmosis occurring in a hydrogen pump has some additional difficulties other than those noted above for a simple hydrogen pump. This type of system uses the fact that every proton passing through the membrane carries water with it, typically about two water molecules per proton. In devices described previously, the hydrogen and water to be purified are fed into the cell together as a solution saturated with hydrogen. Since the solubility of hydrogen in water is low, (&lt;10.sup.-5 M), the current density is limited to a relatively low value. A low current density produces a low water purification rate.
Despite the aforementioned disclosures, there remains a need for membrane and electrode assemblies with improved proton conductivity from the anode to the cathode. Because the sulfonic acid polymers and other proton exchange membranes need water to support proton conduction, there is a need for an improved system for supplying water to the membrane. Additionally, there is a need for membrane and electrode assemblies with improved potential-current density profiles. Finally, there is a need for smaller and less expensive electrolytic cells.